Ca2+ overload is a major cause of myocardial cell damage and cardiac dysfunction in ischemic heart diseases. Studies with gap junction coupling inhibitors and connexin (Cx) 43 knock out mice suggest that Cx43 is important in regulating ischemia/ reperfusion (IR)- induced cellular Ca2+ overload. Cx43 gap junction coupling is regulated by several kinases including mitogen-activated-protein (MAP) kinases. Since ischemic preconditioning improves the recovery of cardiac function after IR-injury and also activates Big- MAPK 1 (BMK1), a member of MARK, we hypothesized that activation of BMK1 induced by preconditioning phosphorylates Cx43, decreases gap junction coupling, and reduces ischemia/ reperfusion-induced cellular Ca2+ overload. Key data in support of our hypothesis include: 1} BMK1 phosphorylates and associates with Cx43, and leads to gap junction uncoupling, 2) Cx43 phosphorylation is increased in cardiac-specific transgenic mice expressing the constitutively active form of the upstream kinase MEK5 (CA-MEK5) which specifically activates BMK1, 3) CA-MEK5 transgenic hearts exhibit a profoundly accelerated recovery of pump function after IR-injury in a Langendorff model. We will study the role of BMK1 and Cx43 phosphorylation in post-ischemic cardiac dysfunction with the following specific aims: Aim 1: Define the role of BMK1 in Cx43 phosphorylation and subsequent Cx43 gap junction uncoupling. Aim 2: Define the role of BMK1 in Cx43 phosphorylation and the effect of Cx43 phosphorylation on IR- induced Ca2+ overload, subsequent calpain activity, and mitochondrial dysfunction. Aim 3: Determine the role of BMK1 and subsequent Cx43 phosphorylation in IR-injury, in vivo, and Aim 4: Investigate the role of BMK1-Cx43 signaling in chemical preconditioning. The broad-based experimental approach should allow us to conclude the importance of BMK1-Cx43 signaling in IR-injury. The long-range goal of this line of investigation is to discover a novel therapeutic strategy for reducing IR-injury.